Field of the Invention
The present application relates generally to a circuit for controlling the brightness of a light emitting diode (LED) and more specifically to a circuit for extending the range over which the LED can be illuminated.
Description of Related Art
With the rising cost of energy, the search continues for lighting solutions that consume less power and operate at a lower overall cost. For a time, compact fluorescent light bulbs, or CFLs as they are commonly known, were believed to be a viable energy efficient solution. One problem with CFLs, however, is that they contain a small amount of mercury (Hg), a potentially dangerous substance, making disposal of the spent CFL bulbs difficult because they cannot simply be thrown in the garbage. Additionally, the mercury from broken CFLs can present a health hazard if not promptly and properly cleaned-up. In response, the Environmental Protection Agency (EPA) has issued guidelines for cleaning up and disposing of CFLs. Because of these issues, an energy efficient alternative to CFLs has been pursued.
Light emitting diodes (LEDs) are small light sources that become illuminated by the movement of electrons through a semiconductor material. Most LEDs belong to one of two categories, low power or high power. LEDs are also increasing in popularity and can be integrated into all sorts of products to provide white and colored light, such as indicator lights, flashlights, light bulbs, and integrated light fixtures. Significant advances have been made in LED technology to produce higher power at lower initial cost to the consumer. Also, LEDs last longer, are more efficient, and produce less heat than traditional incandescent light bulbs. LEDs also contain no mercury.
Circuits for controlling the ON/OFF nature and, to a degree, the brightness of LEDs are known. Conventional circuits for controlling the luminescence, or brightness, of an LED in devices such as lighted GFCIs, electrical receptacles with a nightlight feature, stand-alone nightlights and lighted switches, just to name a few, however, provide limited dimming range. For example, such conventional devices include placing the LED(s) in the collector circuit of a bipolar junction transistor (BJT) and attempting to control the brightness of the LED by controlling the voltage, or current, at the base of the transistor.
Referring to FIG. 3, a conventional circuit 300 for controlling an LED 310 is shown. Specifically, BJT 320 has a base terminal (B), a collector terminal (C) and an emitter terminal (E). Collector terminal (C) of the BJT is connected to the line, or positive, side of a power source at terminal 350 through LED 310, resistor 330 and diode 340. Emitter terminal (E) of the transistor is connected to the neutral, or negative, side of the power source at terminal 360, and base terminal (B) of the transistor is tied to a voltage divider circuit comprising resistor 370 and photo resistor 380.
When the base (B) of transistor 320 is biased with a voltage greater than the base-emitter junction voltage (VBE), current flows through the collector circuit, that is, through resistor 330 and LED 310, to the emitter (E) and ultimately to ground. If the current flowing through the collector circuit exceeds the value necessary to turn ON the LED, LED 310 will illuminate. Because the LED is in the collector circuit and a typical value for the base-emitter voltage, VBE, of a BJT is only 0.6 volts, however, the dimming range of LED 310 in the arrangement shown in FIG. 3 or, in other words, the range by which the brightness of the LED can be controlled, is very narrow. In particular, in accordance with this arrangement, when the base voltage of the transistor is less than 0.6 volts, as compared to the voltage at the emitter (E), which is zero because it is tied to ground, the LED will remain OFF and when the base voltage is equal to or greater than 0.6 volts, the LED is ON. Thus, the brightness of LED 310 is controlled to be either dark or bright, with very little, or no, range in-between.
Thus, according to conventional circuits for controlling an LED lamp such as the circuit shown in FIG. 3, a wide range of brightness is unachievable and such circuits would not be ideal for use in certain devices. For example, certain devices may be used to provide light for people to see in a room where the amount of ambient light varies over the course of the day. Circuits such as the one in FIG. 3 would not be ideal because the LED would either be OFF, when a certain amount of ambient light is present, or ON, when the ambient light drops below that threshold. Accordingly, at times light, or a certain brightness of light, is provided when it is not necessarily needed or desired, and at other times light, or an increased brightness of light, is desired but not provided.
To overcome the problems described above in connection with the conventional circuit shown in FIG. 3, it is has been known to add components to the collector circuit to regulate, or vary, the current flowing in the collector and, thus, in the LED. This technique adds more range of brightness for the LED as determined by the additional circuitry. For example, referring to FIG. 4A a schematic is shown in accordance with this revised conventional approach. Specifically, the schematic shown in FIG. 4A includes a circuit 400 to drive LEDs 410 which are used, for example, to light the area in the vicinity of a conventional electrical receptacle or GFCI device 480 through lens 490, as shown in FIG. 4B. When the ambient light is above a certain level, light sensor 420 reacts to the ambient light level and diode 425 begins to conduct. Sensor 420 is implemented by a light sensing diode and the amount of current conducted by sensor 420 is related to the amount of incident ambient light sensed by the sensor.
As the ambient light increases beyond a predetermined level, a level adjustable by potentiometer 430, the Darlington transistor pair (Q1, Q2) is turned OFF. Specifically, the current flowing through diode 425 pulls down the base of transistor Q1 and transistor Q1, in turn, pulls down the base of transistor Q2. When the ambient light begins to decrease, e.g., as night begins to fall, the current flowing through sensor 420 begins to decrease, accordingly. At some predetermined ambient light level, the current flowing through sensor 420 diminishes to the point where current begins to flow through diode 425 and resistor 427. As a result, transistors Q1 and Q2 are turned ON and collector/emitter current in Q2 flows, thus, energizing LEDs 410.
In the schematic shown in FIG. 4A, a dimmer potentiometer 415 is provided to allow the user to adjust the brightness of the LEDs 410. Sensor 420 and variable resistor 430 function as a voltage divider. Therefore, the voltage presented to diode 425 changes in accordance with the variable resistance of sensor 420.
Although this approach provides additional range in the brightness of the illuminated LED, it also adds complexity and cost to the circuitry and may not be desirable in many applications.
Accordingly, it is desirable to provide a circuit for controlling the brightness of one or more LED lamps over a relatively wide range where the circuit is simple and inexpensive and can be used in a variety of electrical devices.